1. Field of the Invention
The present invention relates to digital modulation/demodulation used for satellite communication, mobile communication, mobile satellite communication, in particular, to a demodulator periodically inserting a known signal into transmitted signals, estimating channel distortion from the known signal, eliminating the channel distortion using the estimated value, and performing coherent detection of the received signal.
2. Description of the Related Art
Recently, in the field of satellite communication, mobile communication, and mobile satellite communication, an investigation of digital modulation/demodulation has been actively performed. Especially, in environments of the mobile communication, a signal is received with fading. Various demodulation systems are examined, which stably operates under such fading environment. Among these systems, a system is remarkably noticed as a system capable to perform absolute coherent detection under fading environment, in which a known signal is periodically inserted into transmitted signals for calibration of channel distortion such as fading, and estimation and compensation of fading distortion is performed based on the known signal.
FIGS. 22 and 23 show conventional demodulation circuits disclosed in, for example, xe2x80x9cRayleigh Fading Compensation Method for 16QAM MODEM in Digital Land Mobile Radio Systemsxe2x80x9d (Sampei, Theses of The Institute of Electronics, Information and Communication Engineers (B-II), J72-II, No. 1, pp7-15 (1989-1)). In FIG. 22, a reference numeral 40 shows a quasi-coherent detecting unit, a reference numeral 50p shows a carrier estimator and 60 shows a data discriminator. In FIG. 23, a reference numeral 51 shows a fading distortion estimator and 52 shows a fading distortion compensator.
In the following, an operation of the above demodulation circuit will be explained.
Here, it is assumed a signal is modulated by QPSK (Quadriphase Phase Shift Keying). A transmitted signal ST(t), modulated by QPSK, is given by the following expression (1).
In expression (1), Re[xe2x80xa2] shows a real part of [xe2x80xa2] and fc shows a carrier frequency. zT(t) shows a transmitted baseband signal and is given by expression (2).
ST(t)=Re[zT(t)exp(j2xcfx80fct)]xe2x80x83xe2x80x83(1)
zT(t)=zp(t)+jxc2x7zQ(t)xe2x80x83xe2x80x83(2)
In case of the above signal transmitted under the fading environment, the faded signal is represented by a narrow-band random complex signal c(t) having fading power spectrum S(f) multiplied to ST(t). The received signal SR(t) faded by an envelope variation c(t) is given by expression (3).
SR(t)=Re[c(t)zR(t)exp(j2xcfx80fct)+n(t)exp(j2xcfx80fct)]xe2x80x83xe2x80x83(3)
In the above expression (3), zR(t) denotes the received baseband complex signal limited by the transmitter and receiver filters. n(t) denotes a white Gaussian noise.
In the following, the receiver will be discussed.
The quasi-coherent detection is performed on the received signal by the quasi-coherent detecting unit 40 with a local oscillator having oscillating frequency of fcxe2x88x92foff. A signal SRB(t) output from the quasi-coherent detecting unit 40 is given by the following expression (4).
sRB(t)=Re└u(t)exp(j2xcfx80fofft)┘
u(t)=c(t)zR(t)+n(t)xe2x80x83xe2x80x83(4)
The carrier estimator 50p obtains cR(t), the estimation value of c(t) from the baseband signal u(t) output from the quasi-coherent detecting unit 40. The carrier estimator 50p also eliminates the distortion caused by c(t) from the received signal. Here, it is assumed that the frequency offset value foff is sufficiently small (foff=0). When the known signal is supposed to be ap, zR(t) becomes zR(t)=ap. Therefore, the received baseband complex signal u(t) is given by the following expression (5).
Accordingly, cR(t), the estimation value of c(t) is obtained by the expression (6).                               u          ⁡                      (            t            )                          =                                            c              ⁡                              (                t                )                                      ⁢                          a              p                                +                      n            ⁡                          (              t              )                                                          (        5        )                                                                                                      c                  R                                ⁡                                  (                  t                  )                                            =                                                1                                      a                    p                                                  ⁢                                  u                  ⁡                                      (                    t                    )                                                                                                                          =                                                c                  ⁡                                      (                    t                    )                                                  +                                                      1                                          a                      p                                                        ⁢                                      n                    ⁡                                          (                      t                      )                                                                                                                              (        6        )            
Based on cR(t) estimated for each known signal, the estimation sequence cRS(t) is represented by the following expression (7) when an insertion interval of the known signal is TR.                                           c            RS                    ⁡                      (            t            )                          =                              ∑                          k              =                              -                ∞                                      ∞                    ⁢                      xe2x80x83                    ⁢                                                    c                R                            ⁡                              (                k                )                                      ⁢                          δ              ⁡                              (                                  t                  -                                      k                    ⁢                                          xe2x80x83                                        ⁢                                          T                      R                                                                      )                                                                        (        7        )            
An estimation of an interpolation sequence cRI(t) out of cRS(t) is, for example, performed by the following way using the interpolation.
In the estimation of fading distortion using the interpolation, the fading distortion of the information signal among the known signal is estimated by the interpolation using the estimation value of the fading distortion obtained by the known signal. Namely, in the fading distortion estimator 51 estimates the fading distortion cR(kxe2x88x921), cR(k), cR(k+1), obtained at t=(kxe2x88x921)TR, kTR, (k+1)TR corresponding to the known signal among the signals output from the quasi-coherent detecting unit 40 as shown in FIG. 24. The fading distortion at t=kTR+(m/NR) TR (NR: the number of symbols corresponding to the insertion interval of the known signal) is estimated, for example, using the second order Gaussian interpolation and given by the following expression (8).                                                                                                                                                                                       c                          RI                                                ⁡                                                  (                                                      k                            +                                                          m                                                              N                                R                                                                                                              )                                                                    =                                                                                                                                  Q                                                              -                                1                                                                                      ⁡                                                          (                                                              m                                                                  N                                  R                                                                                            )                                                                                ⁢                                                                                    c                              R                                                        ⁡                                                          (                                                              k                                -                                1                                                            )                                                                                                      +                                                                                                            Q                              0                                                        ⁡                                                          (                                                              m                                                                  N                                  R                                                                                            )                                                                                ⁢                                                                                    c                              R                                                        ⁡                                                          (                              k                              )                                                                                                      +                                                                                                            Q                              1                                                        ⁡                                                          (                                                              m                                                                  N                                  R                                                                                            )                                                                                ⁢                                                                                    c                              R                                                        ⁡                                                          (                                                              k                                +                                1                                                            )                                                                                                                                                                                                                                                                                        Q                                                      -                            1                                                                          ⁡                                                  (                                                      m                                                          N                              R                                                                                )                                                                    =                                                                        1                          2                                                ⁢                                                  {                                                                                                                    (                                                                  m                                                                      N                                    R                                                                                                  )                                                            2                                                        -                                                          m                                                              N                                R                                                                                                              }                                                                                                                                                                                                                                  Q                    0                                    ⁡                                      (                                          m                                              N                        R                                                              )                                                  =                                  1                  -                                                            (                                              m                                                  N                          R                                                                    )                                        2                                                                                                                                                                Q                    1                                    ⁡                                      (                                          m                                              N                        R                                                              )                                                  =                                                      1                    2                                    ⁢                                      {                                                                                            (                                                      m                                                          N                              R                                                                                )                                                2                                            +                                              m                                                  N                          R                                                                                      }                                                                                      }                            (        8        )            
The fading compensator 52 eliminates the fading distortion from the received signal using the interpolation sequence cRI(k+m/NR) output from the fading distortion estimator 51. Namely, a transmission function h(k+m/NR) for compensating the fading distortion is represented by the following expression (9).
From the expression (9), the fading distortion of the signal output from the quasi-coherent detecting unit 40 is compensated as shown in the expression (10).                               h          ⁡                      (                          k              +                              m                                  N                  R                                                      )                          =                  1                                    c              RI                        ⁡                          (                              k                +                                  m                                      N                    R                                                              )                                                          (        9        )                                                                                                      z                  RE                                ⁡                                  (                                      k                    +                                          m                                              N                        R                                                                              )                                            =                                                h                  ⁡                                      (                                          k                      +                                              m                                                  N                          R                                                                                      )                                                  ·                                  u                  ⁡                                      (                                          k                      +                                              m                                                  N                          R                                                                                      )                                                                                                                          =                                                u                  ⁡                                      (                                          k                      +                                              m                                                  N                          R                                                                                      )                                                                                        c                    RI                                    ⁡                                      (                                          k                      +                                              m                                                  N                          R                                                                                      )                                                                                                          (        10        )            
As described above, the received signal zRE(k+m/NR), of which fading distortion was compensated, can be obtained.
According to the conventional fading distortion estimation and compensation method, the fading distortion of the information signal is estimated by the interpolation using the estimation value of the fading distortion obtained by the known signal. Therefore, in the nonfading channel or Rician fading channel, where thermal noise influences a lot in addition to the fading variation, an error of the estimation value obtained by the known signal becomes large, the fading distortion cannot be compensated properly, which degrades performance of the receiver such as a bit error rate characteristic.
Further, even in the fading channel, thermal noise influences a lot when C/N (carrier power to noise power rate) is low, and the estimation error of the fading distortion estimated by the known signal becomes large. The fading distortion cannot be compensated properly, which degrades the performance of the receiver such as the bit error rate characteristic.
Further, the fading channel and the nonfading channel have opposite characteristics, so that when the performance of the receiver is improved in one of two above channels, the performance of the receiver is degraded in the other channel. It is difficult to embody the receiver having a high performance in wide range from the fading channel to nonfading channel.
The present invention is provided to solve the abovementioned problems. The invention aims to have a demodulator, where the estimation error of the fading distortion is reduced to improve the receiving performance such as the bit error rate characteristic not only in the fading channel, but also in a channel where the thermal noise influences a lot in addition to the fading variation such as the nonfading channel and Rician fading channel, or, in the fading channel where C/N rate is low, the estimation error of the fading distortion is made small to improve the receiving performance such as the bit error rate characteristic.
According to the present invention, a demodulator for estimating a carrier, and for coherent detection using specific symbols inserted in received signals has a fading distortion estimator and a fading distortion compensator using an output of the fading distortion estimator. The demodulator includes:
(1) a modulation component eliminator for filtering the received signals outputted from the fading distortion compensator, to produce estimated-received signals;
(2) an averaging unit for averaging the estimated-received signals to produce a mean (or average) value; and
(3) a signal compensator for compensating the received signals using the mean value.
According to the invention, a demodulation method in a demodulator having a fading distortion estimator and a fading distortion compensator using an output of the fading distortion estimator, includes:
a modulation component eliminating step for filtering received signals including specific symbols from the fading distortion estimator, to produce estimated-received signals;
an averaging step for having a mean value of the estimated-received signals; and
a signal compensation step for compensating the received signals using the mean value of the estimated-received signals.